The present invention relates to a circular polarization controlling optical element for extracting right- or left-handed circularly polarized light from non-polarized light, and to a method of producing the same.
There has conventionally been known, as a circular polarization controlling optical element having the above-described function, an optical element that includes a liquid crystal layer having a cholesteric order (cholesteric regularity) and that reflects, in a specific reflection wave rage, either right- or left-handed circularly polarized light having a wavelength equivalent to the pitch (helical pitch) in the helical structure of the liquid crystal layer and transmits the other circularly polarized light. The term xe2x80x9cliquid crystal layerxe2x80x9d herein used means a layer having the properties of liquid crystal in the optical sense, and includes not only a layer of liquid crystal phase having flowability but also a layer of solid phase obtained by solidifying liquid crystal phase while retaining the alignment of molecules characteristic of the liquid crystal phase.
Such circular polarization controlling optical elements are extensively used in liquid crystal display panels or the like, and are often required to have reflection wave ranges as wide as the entire visible light range.
In the circular polarization controlling optical elements, there has conventionally been known, as a technique for broadening the reflection wave range, such a method that a plurality of liquid crystal layers having reflection wave ranges centered at different wavelengths are laminated. Another known method is such that a cholesteric liquid crystalline material whose helical pitch can be varied stepwise (continuously) is used so that the helical pitch is varied in the direction of thickness (U.S. Pat. No. 5,691,789 and Japanese Patent Laid-Open Publication No. 281814/1994). Further, Japanese Patent Laid-Open Publications No. 319235/1998 and No. 44816/1999 disclose such a method that, after two cholesteric liquid crystalline polymer layers are subjected to contact bonding, the helical pitch is varied stepwise through heat treatment.
In the above-described conventional method in which a plurality of liquid crystal layers having reflection wave ranges centered at different wavelengths are laminated, the entire reflection wave range of the laminate is simply the sum of the reflection wave ranges of the respective liquid crystal layers. It is therefore necessary to laminate a great number of liquid crystal layers if it is desired to obtain a laminate having a reflection wave range that covers the entire visible light range. In this case, however, the influence of reflection of light caused at the interface between each two liquid crystal layers laminated is not negligible, and the laminate is to have poor optical properties.
The above-described method using a cholesteric liquid crystalline material whose helical pitch can be varied stepwise (continuously) is advantageous in that the degree of reflectance of circularly polarized light can be made constant to some extent because the broadening of reflection wave range can be attained by the use of a single liquid crystal layer. In this method, however, it is necessary to incorporate non-crosslinkable liquid crystalline molecules (U.S. Pat. No. 5,691,789) or coloring materials (Japanese Patent Laid-Open Publication No.281814/1994) into the liquid crystalline material. The liquid crystalline material containing such molecules or coloring materials is poor in heat resistance. Moreover, the resulting liquid crystal layer is colored, so that it is poor in optical properties.
Further, in the above-described conventional method in which two cholesteric liquid crystalline polymer layers are subjected to contact bonding and then to heat treatment, the liquid crystalline materials are required to have heat resistance because the heat treatment is carried out at high temperatures. Therefore, types of liquid crystalline materials useful in this method are inevitably limited. In addition, the bonded surfaces of the liquid crystalline polymer layers have been polymerized; this means that the optical interface cannot fully disappear. If such interfaces remain to a large extent, the resulting liquid crystal layer shows poor optical properties.
Furthermore, those liquid crystals that are used in the aforementioned conventional methods are not reactive. It is therefore difficult to fix the structure of the liquid crystal layers after their reflection wave ranges are broadened; if the liquid crystal layers are heated again, they undergo structural changes.
The inventor has made earnest studies in order to overcome the forgoing problems, and, as a result, finally found that it is possible to make the transition from a cholesteric phase in which the helical pitch is intrinsically uniform throughout a liquid crystal layer to such a cholesteric phase in which the helical pitch continuously varies in a liquid crystal layer, by a simple method imparting different degrees of curing to the two surfaces of a liquid crystal layer having a cholesteric order.
The present invention has been accomplished on the basis of the above finding. An object of the present invention is therefore to provide a circular polarization controlling optical element having a broadened reflection wave range without experiencing deterioration of optical properties by interfacial reflection or the like, and to provide a method of producing such an optical element.
Another object of the present invention is to provide a circular polarization controlling optical element showing heat resistance, having optical properties that have been fixed and that will not change even when heated, and to provide a method of producing such an optical element.
A first aspect of the present invention is a circular polarization controlling optical element that includes a cured liquid crystal layer having a cholesteric order in planar alignment, the liquid crystal layer including liquid crystalline molecules and a chiral agent for controlling the helical pitch in the helical structure of the liquid crystalline molecules; wherein the concentration of the chiral agent in the liquid crystal layer linearly varies in the direction of thickness of the liquid crystal layer.
In the first aspect of the present invention, it is preferable that the optical element further includes a substrate for supporting the liquid crystal layer, the substrate having an aligning surface facing the liquid crystal layer, the aligning surface having an aligning power for aligning the liquid crystalline molecules contained in the liquid crystal layer. Further, it is preferable that the liquid crystal layer has a first main surface facing the substrate and a second main surface opposite to the first main surface; and the helical pitch in a portion of the liquid crystal layer, placed on a side of the first main surface, is shorter than that in a portion of the same, placed on a side of the second main surface. On the other hand, it is also preferable that the helical pitch in a portion of the liquid crystal layer, placed on a side of the first main surface, is longer than that in a portion of the same, placed on a side of the second main surface. In addition, it is preferable that the liquid crystal layer further includes a photopolymerization initiator and that the liquid crystalline molecules contained in the liquid crystal layer are at least either one of polymerizable liquid crystalline monomer molecules and polymerizable liquid crystalline oligomer molecules.
A second aspect of the present invention is a method of producing a circular polarization controlling optical element, that includes the steps of: applying a cholesteric liquid crystal solution to a first substrate so as to form an uncured liquid crystal layer, the cholesteric liquid crystal solution having a photopolymerization initiator; and applying ultraviolet light to the uncured liquid crystal layer formed on the first substrate so as to cure the uncured liquid crystal layer, with an exposed surface of the uncured liquid crystal layer, placed opposite to a substrate-side surface facing the first substrate, being exposed to a gaseous atmosphere whose oxygen concentration at a normal pressure is 10% or more.
In the second aspect of the present invention, the gaseous atmosphere is preferably air. Further, in the step of applying the ultraviolet light, it is preferable to gradually decrease the oxygen concentration in the gaseous atmosphere after beginning application of the ultraviolet light. Furthermore, the intensity of ultraviolet light to be applied is preferably from about 10% to about 1% of that of ultraviolet light required to cure, in the gaseous atmosphere, the liquid crystalline molecules contained in the liquid crystal layer while keeping the helical pitch uniform. Further, in the step of applying ultraviolet light, it is preferable to heat the substrate. It is also preferable that the first substrate has an aligning surface facing the liquid crystal layer, the aligning surface having an aligning power for aligning the liquid crystalline molecules contained in the liquid crystal layer. In addition, it is preferable that the method further includes the step of bringing a second substrate made from an oxygen-permeable material into close contact with the exposed surface of the uncured liquid crystal layer, and that, in the step of applying the ultraviolet light, the ultraviolet light is applied to the uncured liquid crystal layer sandwiched between a pair of the substrates while supplying oxygen to the exposed surface of the uncured liquid crystal layer through the second substrate. In this case, it is preferable that the second substrate has an aligning surface facing the liquid crystal layer, the aligning surface having an aligning power for aligning the liquid crystalline molecules contained in the liquid crystal layer.
According to the circular polarization controlling optical element of the present invention, since the helical pitch in the helical structure of liquid crystalline molecules in a liquid crystal layer having a cholesteric order in planar alignment is controlled by linearly changing, in the direction of thickness of the liquid crystal layer, the concentration of a chiral agent in the liquid crystal layer, it is possible to attain the broadening of reflection wave range by a single liquid crystal layer without laminating a plurality of liquid crystal layers; and, moreover, deterioration of optical properties by interfacial reflection can be avoided because optical interfaces are present only in a decreased number. Further, it is not necessary to incorporate non-crosslinkable materials or the like that are usually used to provide a distribution of the concentration of liquid crystalline components, and it is possible to fully fix the structure of liquid crystalline molecules by ultraviolet light; and thus, the cured liquid crystal layer shows heat resistance and has optical properties that have been fixed and that will not change even when heated. In addition, in the step of curing the uncured liquid crystal layer, it is not necessary to heat the layer to high temperatures (150 to 300xc2x0 C.) at which annealing is usually conducted, so that liquid crystalline materials can be selected from a wider range of materials.
Further, according to the method of the present invention, that produces such an optical element, it is possible to make the transition from a cholesteric phase in which the helical pitch is intrinsically uniform throughout a liquid crystal layer to such a cholesteric phase in which the helical pitch continuously varies in a liquid crystal layer, by curing an uncured liquid crystal layer so that the degree of curing on one surface of the cured liquid crystal layer will be different from that on the other surface of the same. It is therefore possible to simply and efficiently produce a circular polarization controlling optical element including a single liquid crystal layer having a broadened reflection wave range, and to well control the width of the reflection wave range of the liquid crystal layer.
Specifically, according to the method of the present invention, one surface of an uncured liquid crystal layer having a cholesteric order in planar alignment is brought into close contact with a substrate so that it will not come into contact with air, while the other surface of the uncured liquid crystal layer is exposed to a gaseous atmosphere whose oxygen concentration at a normal pressure is 10% or more, such as air; and under such conditions, ultraviolet light with a low intensity is applied to the uncured liquid crystal layer to cure the layer. In this case, on the gaseous-atmosphere-side surface of the liquid crystal layer, radical polymerization that is caused by the application of ultraviolet light is hindered due to oxygen in the gaseous atmosphere, so that the liquid crystalline molecules on this side are not easily cured but that those liquid crystalline molecules on the substrate-side surface of the uncured liquid crystal layer are well cured. For this reason, the rate of curing of the liquid crystalline molecules becomes uneven in the liquid crystal layer; and in coincidence with the thus provided distribution of the rate of curing, the concentrations of the liquid crystalline molecules (main component of the cholesteric liquid crystal) and the chiral agent make concentration gradients. In other words, since there is a difference in reactivity (i.e., tendency to get cured) between the liquid crystalline molecules (main component of the cholesteric liquid crystal) and the chiral agent, under the above-described distribution of the rate of curing, the concentrations of the liquid crystalline molecules (main component of the cholesteric liquid crystal) and the chiral agent, which have been uniform in the liquid crystal layer before the application of ultraviolet light, vary in the direction of thickness of the liquid crystal layer. Thus, a cured liquid crystal layer in which the helical pitch on the substrate-side first main surface is different from that on the air-atmosphere-side second main surface is finally obtained.